I don't remember what my previous Xcode Version is, maybe 6.3.2 or 6.4. When I upgrade to Developer Preview Mac OS El Capitan Version, I could not use Xcode.
So I download the Xcode 7 beta 3 (https://developer.apple.com/xcode/downloads/)
Then I installed it, however, My IPhone is iOS 8.4, And it just says unavailable. I follow the instruction and open Windows->Device, It says
"couldn't find the developer disk image"
it doesn't contains the IPhoneOS platform. So I want to install Xcode 6.4, cause it contains iOS 8.4 image, But it says
"Application Xcode can't be used with this OS X version"
I want to combine Xcode 6.4 and Xcode 7 together, So I find a solution: https://forums.developer.apple.com/thread/7698
if you download Xcode 6.4 posted yesterday, then copy Contents>Developer>Platforms>iPhoneOS.platform>DeviceSupport>8.4 (12H141) to the Xcode 7 beta, it seems to work fine. Yes, IPhone Connected.
Finally, I combine El Capitan, iOS 8.4, Xcode 7 and worked well.